On this day in 1904, Sir John Ambrose Fleming invented the electronic valve. Called a tube in the USA, it is a lightbulb-like vacuum cylinder of glass containing a heater, a cathode, and an anode. Once turned on, the cathode heats, and the electrons start buzzing off it towards the anode.

It did not take long for scientists to find multiple everyday applications for Fleming’s valves, which were soon widely used in radios, telephony, radars (including in World War Two fighter planes like Spitfires), sound recording, and even early computers. For instance, in 1944, Tommy Flowers’s Colossus at Bletchley Park used 2,400 valves to tackle the Nazi’s military Lorenz cipher. Two years later, the American military’s ENIAC ballistic calculation computer used 17,468.

COLOSSUS computing machine used to read Nazi codes at Bletchley Park, England, during World War 2Credit:
Pictorial Press Ltd/Alamy

However, it was in audio equipment that valves really took off. They worked efficiently, and produced a pleasing sound. It also became...